The present invention relates to improved inversion carbon blacks as well as a method for their manufacture.
Carbon blacks are used extensively as reinforcement carbon blacks in rubber compounds used in the tire industry. The properties of carbon blacks in this context have an influence, together with the properties of the rubber compounds used, on the performance properties of the completed tires.
The required properties are high abrasion resistance, low rolling resistance, and good adhesion in the case of wet road conditions. The two last properties are influenced essentially by the viscoelastic behavior of the tread compound. In the case of periodic deformation, the viscoelastic behavior can be described by the mechanical loss factor tan .delta., and in the case of elongation or compression, the viscoelastic behavior can be described by the dynamic elongation modulus .vertline.E*.vertline.. Both magnitudes of these values are strongly temperature dependent. The adhesion to wet roads is, in this context, directly correlated with the loss factor tan .delta..sub.0 at approximately 0.degree. C., and the rolling resistance with the loss factor tan .delta..sub.60 at approximately 60.degree. C. The higher the loss factor is at low temperature, the better the adhesion of the tire composition to a wet road usually is. To reduce the rolling resistance, in contrast, a loss factor which is as small as possible at high temperature is required.
The abrasion resistance and the viscoelastic properties, and thus also the loss factor of the tread compounds, are essentially determined by the properties of the reinforcement carbon blacks used. Here, the essential parameter is the specific surface area, particularly the CTAB surface area, which is a measure of the rubber active surface area portion of the carbon black. As the CTAB surface area increases, the abrasion resistance and tan .delta. increase.
Other important carbon black parameters are the DBP absorption and the 24M4-DBP absorption as measured numbers for the starting structure, respecting the residual structure which still remains after mechanically stressing the carbon black, as well as the specific surface area (BET-surface area) of the carbon black as determined according to DIN 66132.
The identified carbon black parameters are dependent on the form of the carbon black particles. In the course of carbon black preparation, there is formed first the so-called primary particles with a diameter of 10 to 500 nm, which then grow into solid three dimensional aggregates. The spatial structure and the particle size distribution as parameters to be measured are exhibited in the precipitation.
For tread compounds, the suitable carbon blacks present a CTAB surface area of 20-190 m.sup.2 /g and 24M4-DBP absorption values of 40-140 mL/100 g.
The average particle diameter of the carbon black aggregate is used for the classification of the carbon blacks according to ASTM D-1765. This classification consists of a four-digit alphanumerical nomenclature, where the first letter (an N or an S) provides information regarding the vulcanization properties, and the first number of the subsequent three-digit number provides information regarding the average particle size. However, this ASTM classification is very rough. Thus, within one ASTM classification range, considerably deviating viscoelastic properties of the tread compounds can occur.
DE 19 521 565 describes inversion carbon blacks which to a large extent satisfy the requirements of low rolling resistance and improved adhesion. These are carbon blacks for which the ratio tan .delta..sub.0 /tan .delta..sub.60 during incorporation into an SSBR/BR rubber compound satisfies the relation EQU tan .delta..sub.0 /tan .delta..sub.60 &gt;2.76-6.7.times.10.sup.-3 .times.CTAB,
and the value of tan .delta..sub.60 is always lower than the corresponding value for ASTM carbon blacks with identical CTAB surface area and 24M4-DBP absorption.
The carbon blacks according to DE 19 521 565 are manufactured according to the furnace carbon black method, which is used today to produce the overwhelming majority of the carbon blacks used in the tire industry. These methods were specially modified for the manufacture of the inversion carbon blacks.
The furnace carbon black method is based on the principle of oxidative pyrolysis; that is, the incomplete combustion of carbon black raw materials in a reactor which is coated with a highly fire-resistant material. As the carbon black raw material, so-called carbon black oils are used, but gaseous hydrocarbons can also be used alone or simultaneously with carbon black oil. Independently of the special construction design of the reactors, three zones can be distinguished in the carbon black reactor, which correspond to the different steps of the carbon black production. The zones are present successively along the reactor axis, and the reaction medium flows through them in succession.
The first zone, the so-called combustion zone, essentially comprises the combustion chamber of the reactor. Here a hot combustion chamber exhaust gas is generated, by burning a fuel, as a rule a hydrocarbon fuel, with an excess of preheated combustion air or other oxygen-containing gases. Natural gas is predominately used today as the fuel, but it is also possible to use liquid hydrocarbons such as heating oils. The combustion of the fuel usually occurs under conditions with an excess of oxygen. According to the book "Carbon Black" second edition, Marcel Dekker Inc., New York, 1993, page 20, it is very important, for the purpose of obtaining optimal use of the energy, that the conversion of the fuel to carbon dioxide and water occurs as completely as possible in the combustion chamber. In this process, the excess air promotes the complete conversion of the fuel. The fuel is usually introduced by means of one or more combustion lances into the combustion chamber.
The K factor is frequently used as an index number to characterize the excess air. The K factor is the ratio of the quantity of air required for the stoichiometric combustion of the fuel to the quantity of air which is in fact fed to the combustion. A K factor of 1 thus means that the combustion is stoichiometric. If there is an excess of air, the K factor is smaller than 1. Usually K factors of 0.3-0.9 are used.
In the second zone of the carbon black reactor, called the reaction zone, carbon black formation takes place. For this purpose, the carbon black raw material is injected and admixed in the hot waste gas stream. With respect to the oxygen quantity which is not completely reacted in the combustion zone, there is an excess hydrocarbon quantity introduced into the reaction zone. Therefore, under normal conditions, carbon black formation starts here.
Carbon black oil can be injected into the reactor in different manners. For example, an axial oil injection lance, or one or more radial oil lances which are arranged on the circumference of the reactor, in a plane which is vertical with respect to the direction of flow, are suitable. A reactor can have several planes with radial oil lances, along the direction of flow. At the tip of the oil lances, either spray or injection nozzles are provided, by means of which the carbon black oil is admixed in the waste gas stream.
In the case of simultaneous use of carbon black oil and gaseous hydrocarbons, such as, for example, methane, as the carbon black raw material, the gaseous hydrocarbons can be injected separately from the carbon black oil through a special set of gas lances into the hot waste gas stream.
In the third zone of the carbon black reactor, called the termination zone (quenching zone), carbon black formation is stopped by a rapid cooling of the carbon black-containing process gas. This process prevents any undesired secondary reactions. Such secondary reactions would lead to porous carbon blacks. The reaction is usually stopped by spraying in water using appropriate spray nozzles. Usually there are several points along the carbon black reactor for water spraying, for example, for "quenching" so that the residence time of the carbon black in the reaction zone can be varied. In an in-line heat exchanger, the residual heat of the process gas is used to preheat the combustion air.
A multitude of different reactor forms has become known. The different variants concern all three reactor zones, but a particularly high number of embodiment variants exist for the reaction zone and the arrangement of injector lances for the carbon black raw material. Modern reactors usually have several oil injection lances, distributed around the circumference of the reactor and also along the reactor axis. The carbon black oil quantity, distributed over several individual streams, can be better admixed in the stream of hot combustion waste gases flowing out of the combustion chamber. By means of introduction points distributed spatially in the direction of the flow, it is possible to stagger the oil injection over time.
The primary particle size, and thus also the normally easily determinable specific carbon black surface area, can be controlled by the quantity of carbon black oil injected into the hot waste gas. When the quantities and the temperatures of the waste gases generated in the combustion chamber are kept constant, the quantity of carbon black oil alone is responsible for the primary particle size, which relates to the specific carbon black surface area. Larger quantities of carbon black oil lead to coarser carbon blacks with lower specific surface areas than smaller quantities of carbon black oil. Simultaneously with a change in the quantity of carbon black oil, there is a change in the reaction temperature; since the sprayed carbon black oil lowers the temperature in the reactor, larger quantities of carbon black oil mean lower temperatures, and vice versa. From this it is possible to derive the relationship between the carbon black formation temperature and the specific carbon black surface area, in relation to primary particle size, which was described in the book "Carbon Black" cited above, on page 34.
If the carbon black oil is distributed from two different injection points, which are separately located along the reactor axis, then, in the first upstream location, the quantity of residual oxygen still contained in the combustion chamber waste gas is still in excess with respect to the sprayed carbon black oil. Thus, carbon black formation occurs at this point with a higher temperature compared to the subsequent carbon black injection points, that is, in the first injection point, the formed carbon blacks have finer particles, and present a higher specific surface area, than at a subsequent injection point. Each additional injection of carbon black oils leads to additional temperature drops and to carbon blacks with larger primary particles. Carbon blacks prepared in this manner thus present a widening of the particle size distribution curve and, after incorporation in a rubber, they present a different behavior than carbon blacks with a very narrow monomodular particle size spectrum. The wider particle size distribution curve leads to a lower loss factor of the rubber compound, that is to a low-hysteresis, and therefore the expression low-hysteresis (lh) carbon blacks is used. Carbon blacks of this type, or methods for their manufacture, have been described in the European Patents EP 0,315,442 and EP 0,519,988.
The conventional methods are thus able to produce, by means of the spraying devices for carbon black oil positioned at intervals along the reactor axis, carbon blacks with a wider particle size distribution curve, which impart a lower rolling resistance to rubber compounds in which they have been incorporated.
For the manufacture of the inversion carbon blacks, the furnace carbon black method was modified in another manner. Whereas the conventional furnace carbon black methods are intended to obtain as complete as possible a combustion of the fuels in the combustion chamber, more particularly in the combustion zone, the method according to DE 195 21 565 for the manufacture of inversion carbon blacks is based on the formation of hydrocarbon nuclei as a result of the incomplete combustion of the fuel in the combustion zone. The nuclei are then transported with the hot waste gas stream into the reaction zone, where a nucleation-induced carbon black formation is initiated with the added carbon black raw material. The intended incomplete combustion of the fuel, however, does not mean that the fuel is burned in a less than stoichiometric amount of oxygen. Rather, the method according to this invention also starts with an excess of air or oxygen-containing gases in the combustion chamber. As with conventional carbon blacks, K-factors of 0.3-0.9 can be used in this process.
In order to generate carbon black nuclei in spite of the excess air, different routes can be engaged according to DE 195 21 565. In a preferred variant of the method, liquid hydrocarbons are used as the starting fuel, which are then burned instead of natural gas in the combustion chamber of the reactor with an excess of air or oxygen-containing gases. Liquid hydrocarbons burn more slowly than gaseous hydrocarbons, because they first must be converted into the gaseous form, that is they must be vaporized. In spite of an excess of oxygen, liquid hydrocarbons can therefore be not only burned, but also used for the production of hydrocarbon nuclei which--if sufficient time is available and the temperature is sufficiently high--also burn, or they can grow to form larger carbon black particles if a rapid cooling is applied. The nucleation induced carbon black formation is based on the fact that the nuclei which are formed during the combustion of liquid hydrocarbons with an excess of oxygen are immediately brought in contact with the carbon black oil, and thus nucleus growth is initiated.
An additional variant of the method according to DE 195 21 565 uses natural gas as the fuel. Nucleation is achieved by selecting an outflow rate for the gas out of the combustion lance(s) which is so low that a poor admixture of the natural gas in the hot stream of combustion air is intentionally achieved. It is known that carbon black nuclei form in cases of poorly mixed flames, and the term luminous flames is used because of the lighting up of the particles which form. In this procedure, as in the combustion of liquid hydrocarbons, it is important for the formed nucleus to be brought into contact, immediately with their formation, with the carbon black oil. If one uses a larger combustion chamber, or combustion zone, to effect the conversion of the nuclei with the oxygen present in excess in the combustion zone, one thereby allows a complete combustion in the combustion zone of the carbon black reactor, and thus no nucleation-induced carbon black formation occurs.
Both described variants can also be combined. In that case, the liquid hydrocarbons and the natural gas, or other gaseous components, are simultaneously fed in appropriate ratios into the combustion zone. It is preferred to use oils, for example, the carbon black oil itself, as the liquid hydrocarbon.
The method according to DE 195 21 565 thus resides in using liquid and/or gaseous hydrocarbons as fuels in the combustion zone, in which the oxygen, with respect to the hydrocarbons used, is present in excess. This ensures that carbon black nuclei are formed, for example, due to the insufficient residence time of the liquid hydrocarbons or due to an insufficient mixing of the gaseous hydrocarbons with the combustion air. These carbon black nuclei are then brought into contact in the reaction zone, immediately after their formation, with the carbon black raw material, which is used in excess with respect to the quantity of oxygen. Cooling of the resulting carbon black reaction gas mixture follows by the introduction through nozzles of water into the termination zone, and the further processing of the carbon black so formed in the usual manner.
According to DE 195 21 565 the fuel plays an important role in carbon black formation; it is referred to as the primary carbon black raw material below. The carbon black raw material which must be admixed in the reaction zone is accordingly referred to as the secondary carbon black raw material, and, in terms of quantity, it accounts for the majority of the carbon black which forms.
The inversion carbon blacks according to DE 195 21 565 impart to the carbon black mixtures, compared to conventional carbon blacks, a reduced rolling resistance and a comparable adhesion under wet conditions. Furthermore, ATM (atomic force microscopy) examinations revealed that the inversion carbon blacks present a significantly rougher surface than the corresponding standard ASTM carbon blacks, which results in an improved binding of the rubber polymers to the carbon black particles (see W. Gronski et al., "NMR Relaxation, A Method Relevant for Technical Properties of Carbon Black-Filled Rubbers, International rubber conference 1997, Nuremberg, page 107). The improved binding of the rubber polymer leads to a reduction in the rolling resistance.
Examinations concerning the abrasion of rubber compounds using inversion carbon blacks have shown that these carbon blacks impart to the rubber compounds an improved abrasion resistance with lower exposure to loads. In the case of high loads, for example, in trucks tires, these rubber compounds present an increased abrasion.
An object of the present invention, therefore, is to provide improved inversion carbon blacks which are characterized particularly by a reduced abrasion under high loads.